As the Democratic presidential candidates prepare for Saturday's debate in Des Moines, they should consider themselves fortunate in one respect. They no longer have to outdo each other to prove how tough they are on crime. Gone are the days when then-Gov. Bill Clinton left the campaign trail in 1992 to supervise the execution of a brain-damaged Ricky Ray Rector. In fact, we've come something like full circle. The discussion now is not how punitive the criminal justice system should be, but how it can be reformed.

Call it the safety dividend. Crime is half of what is was 25 years ago and about 15 percent less than it was just five years ago. The public is no longer in fear. This new state of affairs has prompted a re-examination by both parties of the harsh sentences handed out under the current system. In fact, a bipartisan bill to reduce some sentences is now wending its way through Congress and has a good chance of passage.

While reducing federal prison sentences is certainly a welcome first step, it does little to confront the central problem of the criminal justice system: the 2.3 million people behind bars across the nation. Today, there are five times as many people incarcerated than there were in 1970. If the prison population were a state, it would be the 36th largest -- bigger than Delaware, Vermont, and Wyoming combined. The unnecessarily large prison population is also a major contributor to economic inequality. One study found that if not for the rise in incarceration, the poverty rate would fall by as much as 20 percent.

Perhaps most importantly, mass incarceration is not necessary to fight crime. States such as Texas, California and New Jersey have all simultaneously reduced their prison populations and their crime rates. In the foreword for a study we published last year, Nobel Prize-winning economist Joseph E. Stiglitz wrote that "At today's high incarceration rates, continuing to incarcerate more people has almost no effect on reducing crime."

To their credit, each of the Democratic candidates has offered suggestions for criminal justice reform. Yet none of them really offer much in the form of concrete policies that would truly bring about the systemic changes needed to end mass incarceration.

Hillary Clinton has explicitly called for "ending the era of mass incarceration." She's offered policies to reclassify marijuana to a less dangerous drug, thereby reducing some drug sentences and has called for an end to racial profiling.

And, Martin O'Malley's proposals focus on issues that skirt the problem but do not fully address it: from creating national guidelines for police use of force to reducing solitary confinement to ending the death penalty. These are important issues but do little to reduce the current prison population.

Like O'Malley and Clinton, Bernie Sanders has called for reforming mandatory minimums while advocating for treatment, not prison, for those with addiction problems. His central proposal would ban government contracts with privately-owned prisons, aiming to curb an "over-incentive to arrest, jail and detain, in order to keep prison beds full." But since only eight percent of prisoners are in privately-held facilities, even if they were all eliminated, the impact on the prison population would be modest.

What each of the candidates miss is the enormous power the federal government - and the next President - wields over the policies of states, which hold 86.5 percent of the nation's prisoners. In fact, it was federal largesse that helped create mass incarceration. In 1994 Congress passed the Violent Crime Control and Law Enforcement Act, the largest crime bill in U.S. history. Among other things, the bill set aside $12.5 billion (about $20 billion today's dollars) for prisons. The result was more fuel for a prison construction boom. At one point in the mid-1990's, a new prison opened an average of every 15 days.

With growing bipartisan agreement around criminal justice reform, all of the presidential candidates, not just Democrats, should be willing to take bold action. What's needed is major legislation to meet the criminal justice needs of the 21st century, not incremental tinkering with an overly-punitive system designed to combat a crime rate that was almost twice as high as it is today.

A crime bill to meet today's needs would contain the following:

A ten-year, $20 billion federal grant program with goals states must meet to be eligible for funds.

To receive money, states must reduce their prison population by 7 percent over a three-year period without an increase in crime.

Funds will be allocated according to a clear methodology including factors such as population size.

States must invest the federal grant funds in programs proven to reduce crime and incarceration.

Such a "Reverse Mass Incarceration Act" would allow the next President to create a systemic change across the country. It would leverage the billions of federal dollars now spent to subsidize mass incarceration in states to instead reduce today's unprecedented incarceration rates while also keeping crime down.

When it comes to criminal justice, the nation is at a crossroads. There is widespread agreement that too many are in prison for too long. Saturday night would be an ideal time for one of the candidates to begin demonstrating leadership on this issue.